LAKE    SUPERIOR   REGION.  167 
north  of  Crystal  Falls  the  Mansfield  formation  was  being  deposited  in 
the  southeastern  part  of  the  district.  This  activity  continued  there 
through  the  time  in  which  the  Groveland  formation  was  being  depos- 
ited in  other  parts  of  the  district. 
From  the  foregoing  it  appears  that  the  Hemlock  formation  in  the 
western  part  of  the  district  is  equivalent — 
1.  East  of  the  western  Archean  oval,  to  the  Hemlock  volcanics 
found  there  and  the  overlying  Groveland  formation. 
2.  At  Michigamme  Mountain,  to  the  Mansfield  slates  and  the 
Groveland  formation. 
3.  In  the  Mansfield  area,  to  the  Mansfield  slates  and  the  Hemlock 
volcanics  occurring  there. 
4.  In  the  southeastern  part  of  the  district,  to  the  Mansfield  and 
Groveland  formations. 
The  replacement  of  an  iron-bearing  formation  by  the  great  vol- 
canic formation  just  described  is  exactly  paralleled  in  the  Upper 
Huronian  rocks  on  the  Penokee  iron-bearing  series,  where  the  pure 
iron-bearing  formation  is  replaced  at  the  east  end  of  the  district  by  a 
great  volume  of  volcanic  rocks  intercalated  with  slates  and  containing 
bunches  of  iron- formation  material. 
Following  the  deposition  of  the  Lower  Huronian  series  the  region 
was  raised  above  the  sea  and  eroded  to  different  depths  in  different 
places.  In  the  Felch  Mountain  range  the  only  formations  above  the 
Randville  dolomite  are  a  thin  bed  of  slate  and  the  Groveland  iron 
formation.  In  the  northeastern  part  of  the  district  only  a  thin  belt 
of  iron- formation  rocks  remains.  In  the  central  and  western  parts  of 
the  district  there  is  a  great  thickness  of  volcanics.  This,  however, 
does  not  imply  a  difference  of  erosion  equal  to  the  difference  in  thick- 
ness of  these  rocks,  for  doubtless  when  the  volcanics  were  built  up 
there  was  contemporaneous  subsidence,  so  that  at  the  end  of  Lower 
Huronian  time  there  may  have  been  little  variation  in  the  elevation 
of  the  upper  surface  of  the  series,  but  very  great  differences  in  its 
thickness. 
Upper  Huronian. — After  the  Lower  Huronian  series  was  deposited 
the  district  was  raised  above  the  sea,  may  have  been  greatly  folded, 
and  different  parts  were  eroded  to  different  depths. 
Following  the  earth  movements  and  erosion,  the  waters  for  some 
reason  advanced  over  the  district,  and  the  Upper  Huronian  series  was 
deposited.  The  basal  horizon  was  a  conglomerate,  which  has,  how- 
ever, very  different  characters  in  different  parts  of  the  district. 
In  the  eastern  half  were  Archean  rocks,  the  Sturgeon  quartzite, 
the  Mansfield  slate,  and  the  Groveland  iron  formation.  Upon  these 
was  deposited  a  sandstone  which  locally  was  very  ferruginous.    This 
